No one can dispute the value of knowledge about the three-dimensional structure of proteins in defining their mechanisms of action. We are proposing to provide structural work on key proteins in defining their mechanisms of action. We are proposing to provide structural work on key proteins involved in the regulation of smooth muscle contraction, namely tropomyosin, caldesmon, and calponin. These proteins are the subject of extensive biochemical research in other parts of this program project proposal; structural characterization will be crucial in the interpretation of much of this work and is an essential component in a thorough investigation of these molecules. We propose to use techniques of molecular biology, biochemistry, and X-ray crystallography to provide a structural basis for interpreting the functional data and hence allow an understanding of smooth muscle regulation at the molecular level. At each opportunity, comparisons will be made to striated muscle regulation. Specifically we propose three major objectives, solution of the crystal structure of smooth muscle tropomyosin, structural studies on caldesmon and its fragments and their binding to tropomyosin, and crystallization and structural analysis of calponin and its interactions with tropomyosin. Solution of the proposed structures will provide the framework needed to interpret critically the protein-protein, domain mapping, and site-directed mutagenesis experiments proposed for caldesmon, calponin and tropomyosin. In addition, we anticipate that the availability of high resolution structural information will generate new insight into how to design more informative experimental strategies.